This invention relates generally to mobile communications systems such as cellular telephone and satellite systems, and more particularly the invention relates to the paging of mobile users in such systems.
Mobile satellite systems and cellular systems are generally star configured, with all communications going through a ground station. The outbound ground to mobile link is referred to as the Forward Link (FL). Satellite systems usually employ multibeam antennas providing coverage which has some similarity to a cellular system. The FL in each beam is received by all active user terminals (UT) within the beam. The FL generally consists of common control channels which are monitored by all users, user associated control channels, traffic channels, and synchronization overhead.
Terrestrial cellular telephone systems such as AMPS, GSM, and IS-95 all include forward link paging channels where "paging" is defined as "the act of seeking a mobile station when an incoming call from the land station has been placed to it". See Lee W. C. Y., Mobile Cellular Telecommunication Systems, McGraw-Hill Book Company, 1989. In each case, the Paging Channel is similar in structure to a control channel and carries a message containing the address of the user terminal (UT) that is being called. In the case of IS-95, the paging channel can operate at rates of up to 9600 bps.
Satellite systems differ from terrestrial system in a number of important ways--one of the most important is that forward link power is a scarce and precious resource. Link margins are necessarily limited and it is desirable to keep the overhead due to paging to a minimum.
There are a number of global and regional mobile satellite systems designed to work with handheld UT's that are presently in the proposal or development stage. These include Odyssey, Globalstar, ICO and Thuraya, among others.
Typically, a handheld UT which is available to receive incoming calls would monitor a common control channel used for paging and listen for its user address. The paging may be organized in "paging groups" so that the UT only listens to pages at certain times (e.g. once every 2 seconds) rather than continuously. This allows the terminal to shut down part of the time and save battery power. If the UT detects a page, it responds and call set-up proceeds. The paging operation is usually done at the normal control channel data rate and requires an Eb/No of from 3 to 9 dB depending on the amount of coding, the type of modulation employed, and the desired performance. The total received energy in the page is: EQU E.sub.T =mP.sub.s T.sub.b ;
where:
m=number of bits in the paging message;
P.sub.s =received signal power; and
T.sub.b =data bit period.
Some satellite systems are considering a "deep paging" function. The purpose of deep paging would be to alert a shadowed user, who could not detect the ordinary traffic or control channels due to signal attenuation, that there was an incoming call. The user could then move to a more favorable location, such as a window if he were inside a building. Deep paging would typically take place at signal levels of 25 dB or more below nominal. The deep paging message would typically be sent at a very low data rate to accommodate the low signal level. This makes coherent detection problematic due to UT phase noise, as well as channel coherence times. Noncoherent detection is much more attractive but is not as power efficient.
The present invention is directed to a paging system with increased power efficiency and having the robustness of noncoherent detection.